Structural stability in nanocrystalline ZnO

The grain-size effect on phase transition induced by pressure in ZnO nanocr ystals has been investigated by in situ high-pressure synchrotron radiation X-ray power diffraction optical and electrical resistance measurements. Th e transition pressure of the B4-to-B1 phase transformation for 12 nm ZnO is found to be 15.1 GPa while it is 9.9 GPa for bulk ZnO Three components: th e ratio of the volume collapses, the surface energy difference, and the int ernal energy difference, governing the change of transition pressure in nan ocrystals, are uncovered. The enhancement of transition pressure in ZnO nan ocrystals as compared with the corresponding bulk material is mainly caused by the surface energy difference between the phases involved. The high-pre ssure B1 ZnO phases net metallic in the pressure range up to 18 GPa at room temperature.